This application claims priority under 35 U.S.C. § 119(a) on patent application Ser. No. 2005-005375 filed in Japan on Jan. 12, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to solid state relays (“SSRs”) to which protective elements can easily be connected as a measure for protecting against surges, to electronic devices equipped with such SSRs, and to methods for manufacturing such SSRs.
SSRs which use semiconductor elements as switching elements for switching on and off a power supply to a load are well known.
SSRs are required to have a high breakdown voltage as they directly control switching on and off the power supply. For this reason, SSRs are equipped with high-breakdown-voltage, high-power semiconductor elements such as triads as load-controlling power elements. As the need arises to ensure isolation between a primary (input) side which acts as a control system with a relatively low voltage and a secondary (output) side with a relatively high voltage which handles a large power supply, optical coupling (photo couplers) which can easily achieve input/output isolation are used for transmission of signals between the input and output sides.
This kind of SSR achieves good input/output isolation and control, and is therefore often used in electronic devices such as power devices, home appliances, inverter controllers.
FIG. 10 is a partially cut-away perspective view showing an example of a conventional SSR.
SSR 50 comprises a light-emitting diode 51 mounted as a light-emitting element on an input side, and a photo-triad 52 and a triad 53, mounted on an output side as a light-receiving element and as a load-controlling power element, respectively.
The light-emitting diode 51 is die-bonded using silver paste or the like and is wire-bonded using gold wire or the like to an input-side lead frame 55. The photo triad 52 is die-bonded using a silver paste or an insulating paste such as a polyimide resin, and wire-bonded using gold wire or the like to an output-side lead frame 56.
The light-emitting diode 51 and the photo triad 52 are disposed facing each other, configured to allow transmission of signal light there between. The inner sides of the light-emitting diode 51 and the photo triad 52 are sealed with resin, using an inner resin sealing portion 57 of white translucent resin, and sealed with resin, using a black resin sealing portion 58 from its outside.
Like the photo triad 52, the triad 53 is die-bonded and wire-bonded to an output-side lead frame 56 and driven by a current signal from the photo triad 52 when this last receives a signal light from the light-emitting diode 51, thereby constituting a switching element (not shown) which controls switching on and off the power supply to the load.
FIG. 11 is a wiring diagram of the conventional SSR shown in FIG. 10. This type of conventional SSR is disclosed in JP H7-106629A, for example.
The SSR 50 is provided, on the input side, with an anode terminal Ta to which is connected an anode 51a of the light-emitting diode 51 and a cathode terminal Tk to which is connected a cathode 51k of the light-emitting diode. On the output side are provided a first output terminal Tp1 to which is connected a first electrode 53f of the triad 53 and a second output terminal Tp2 to which is connected a second electrode 53s of the triad 53. A first electrode 52f of the photo triad 52 is connected to the first output terminal Tp1 and a second electrode 52s of the photo triad 52 is connected to a gate electrode 53g of the triad 53.
This type of SSR 50 is used in electronic devices such as power devices. However, the growing diversification of the environment in which electronic devices are used has meant such devices are often exposed to external current and voltage surges, leading to frequent occurrence of breakdown of the photo triad 52 by a surge. Destruction of the photo triad 52 by a surge entails the risk of destroying the electronic device itself, creating a need for measures against surges in order to stabilize and improve the reliability of operation of the SSR 50.
FIG. 12 is a wiring diagram of a solid state relay incorporating a countermeasure against surges. FIG. 13 is a schematic cross-sectional view of a case in which a protective element (see below) shown in FIG. 12 is applied to the conventional solid state relay shown in FIG. 10.
An SSR 60 (FIG. 12) which incorporates a countermeasure against surges is identical to the conventional example shown in FIG. 11 as regards the basic circuit configuration, but differs in that a built-in resistor R3 is connected between the first electrode 52f of the photo triad 52 and the first electrode 53f of the triad 53 as a protective element.
The SSR 50 shown in FIG. 13 is equipped with the light-emitting diode 51 on the input-side lead frame 55 and the photo triad 52 and the triad 53 on the output-side lead frame 56 facing the input-side lead frame 55, the light-emitting diode 51, photo triad 52, triad 53, input-side lead frame 55, and output-side lead frame 56 being sealed with resin by the inner resin sealing portion 57 and sealed with resin by the resin sealing portion 58 from its outside.
On the SSR 50, on which the input-side lead frame 55 and the output-side lead frame 56 are opposed and which is sealed with resin (using transfer molding) by the inner resin sealing portion 57 and the resin sealing portion 58, the input-side lead frame 55 and the output-side lead frame 56 extend over almost the entire area of the inner resin sealing portion 57 and the resin sealing portion 58 in order to raise heat diffusion.
If the SSR 50 were equipped with the built-in resistor R3 (indicated by the broken lines) as a protective element thicker than the chip thickness of the light-emitting diode 51, the photo triad 52, and the triad 53, the shortest electrical distance between the input-side lead frame 55 and the output-side lead frame 56 would change from a conventional distance d5 to a distance d6. In other words, the breakdown voltage would drop and the risk of dielectric breakdown would rise because of the shorter electrical distance between the input-side lead frame 55 and the output-side lead frame 56. Accordingly, a need arises to increase the distance between the input-side lead frame 55 and the output-side lead frame 56 in order to maintain the insulation between the input-side lead frame 55 and the output-side lead frame 56.
However, increasing the distance between the input-side lead frame 55 and the output-side lead frame 56 creates the need to raise the luminance of the light-emitting diode 51 as a light-emitting element and the sensitivity of the photo triad 52 as a light-receiving element in order to achieve accurate light transmission.
In order to raise the luminance of the light-emitting diode 51, the current flowing to the light-emitting diode 51 must be increased, but this is not preferable from the perspective of power consumption, heat and so on. In order to raise the sensitivity of the photo triad 52, the surface area of the photo triad 52 needs to be enlarged, but this is not preferable from the perspective of compactness, current leakage, and so on. Accordingly, mounting the built-in resistor R3 which is thicker than the chip thickness of the light-emitting diode 51, the photo triad 52, and the triad 53 is unrealistic and difficult to realize.
The present invention was conceived with the above-mentioned circumstances in mind, and its object is to provide a highly-reliable SSR capable of preventing breakdown of a light-receiving element by a surge, electronic devices provided with such an SSR, or a manufacturing method for manufacturing such an SSR.